Tubular medical endoprostheses

ABSTRACT

A tubular prosthesis device for use within the body. The device includes, a metal filament material formed of metal outer member having an exposed outer surface and a core within the extended outer member formed of a different metal than the outer member. The core is secured within and substantially enclosed by the outer member. The device can be reduced to a small size for introduction into the body lumen and expandable to a sufficiently large size to engage the wall of the body lumen. Stents formed of composite wires are shown.

CROSS-REFERENCE TO RELATED APPLICATION This application is acontinuation-in-part of U.S. Ser. No. 07/861,253, filed Mar. 31, 1992.FIELD OF THE INVENTION

[0001] This invention relates to tubular endoprostheses to be usedinside the body.

BACKGROUND OF THE INVENTION

[0002] Tubular endoprostheses such as medical stents are placed withinthe body to perform a function such as maintaining a body lumen open,for example, a passageway occluded by a tumor or a blood vesselrestricted by plaque. Tubular endoprostheses in the form of grafts areused to substitute for or reinforce a lumen, such as the aorta or otherblood vessels which have been weakened, e.g., by an aneurysm.

[0003] Typically, these endoprostheses are delivered inside the body bya catheter that supports the device in a compacted or otherwisereduced-size form as it is transported to the desired site. The size isparticularly small when a percutaneous insertion technique is employed.Upon reaching the site, the endoprosthesis is expanded so that itengages the walls of the lumen.

[0004] The expansion mechanism may involve forcing the endoprosthesis toexpand radially outwardly, for example, by inflation of a ballooncarried by the catheter, to plastically deform and fix the device at apredetermined expanded position in contact with the lumen wall. Theexpanding means, the balloon, can then be deflated and the catheterremoved.

[0005] In another technique, the endoprosthesis is formed of a highlyelastic material that can be reversibly compacted and expanded. Duringintroduction into the body, the endoprosthesis is restrained in thecompacted condition and upon reaching the desired site for implantation,the restraint is removed, enabling the device to self-expand by its owninternal elastic restoring force.

[0006] In many cases, X-ray fluoroscopy is used to view anendoprosthesis within the body cavity to monitor placement andoperation. The device may also be viewed by X-ray film after placementfor medical follow-up evaluation. To date, the requirement forradiopacity has limited the construction of devices to certain materialswhich in turn has limited the features attainable for particularapplications and the available insertion techniques.

SUMMARY OF THE INVENTION

[0007] In the invention, metal such as in the form of wire or filamentor the like is used for constructing tubular medical endoprosthesis suchas stents. Desirable attributes of these wires vary with the stentapplication, but include properties such as stiffness, tensile strength,elasticity, radiopacity, weldability, flexural life, conductivity, etc.These properties are hard to find in conventional wires. According tothe invention, desired properties are achieved by creating a multiplemetal coaxial construction. For example, it may be very desirable tohave high radiopacity along with elasticity and strength. This isaccomplished by combining a radiopaque metal with an elastic metal.Although it is possible to put either metal on the inside or outside, itis preferable to put the dense radiopaque material (e.g., tantalum) onthe inside (core) since dense materials are generally less elastic andthe elastic material (e.g., titanium or nickel-titanium alloy) on theoutside (clad). The clad or “skin” of the wire will undergo moredeformation in bending than the core, so the elastic component is bestpositioned at the skin.

[0008] Thus, an aspect of the invention is a metal stent device with atleast a portion to be used within the body having properties that can betailored to a particular application. The portion within the body isformed of preferably two or more dissimilar metals joined together toform a unitary member. Typically, each metal contributes a desirableproperty to the device which is not substantially impaired by thepresence of the other metal. In particularly preferred devices, onemetal provides enhanced radiopacity. In these embodiments, the stent iscomprised of a metal outer member having a predetermined density and anexposed outer surface and a core including a metal having a densitygreater than the outer member to enhance radiopacity. The core issecured within and substantially enclosed by the outer member.Preferably, the stent is configured such that the mechanical properties,for example, the elastic properties, of the metal forming the outermember are affected by the core to a desired degree so that the stenthas a desired overall performance suitable for its intended use.Preferably, the mechanical properties of the outer member dominate theproperties of the stent yet the radiopacity of the member issubstantially enhanced by the denser core. The invention also allowsincreased radiopacity of the stent without adversely affecting and insome cases improving other important properties such as thebiocompatibility, small size or other performance characteristics. Theseperformance advantages can be realized by proper selection of thematerial of the outer member and core, their relative size, andgeometrical configuration. The particular performance characteristics tobe achieved are dictated by the stent application.

[0009] The term “metal” as used herein includes electropositive chemicalelements characterized by ductility, malleability, luster, andconductivity of heat and electricity, which can replace the hydrogen ofan acid and forms bases with the hydroxyl radical and including mixturesincluding these elements and alloys. Many examples are given below.

[0010] An aspect of the invention features a tubular prosthesis devicefor use within the body. Forming the tubular endoprosthesis is a metalfilament material comprised of a metal outer member of extended lengthhaving an exposed outer surface and a core within the extended outermember formed of a different metal than the outer member. The core issecured within and substantially enclosed by the outer member. Thedevice is capable of reduction to a small size for introduction into thebody lumen and expandable to a sufficiently large size to engage thewall of the body lumen.

[0011] In some preferred embodiments, the outer member and core are suchthat the endoprosthesis is elastic and capable of radial reduction insize without plastic deformation to the small size for introduction tothe body and self-expandable by an internal elastic self-restoring forceto the large size for engaging the wall of the lumen.

[0012] In other embodiments, the outer member and core are such that theendoprosthesis is plastically deformable; it is formed of small size forintroduction into the body and expandable by plastic deformation to thelarge size for engaging the wall of the lumen.

[0013] Various embodiments of the invention may also include one or moreof the following features. The device is formed into the tubular shapeby knitting of the wire or filament into loosely interlocked loops ofthe filament. The metal of the core has a density greater than the metalof the outer member of the device. The cross sectional dimension of thefilament is about 0.015 inch or less. The cross-sectional dimension ofthe filament is about 0.006 to about 0.0045 inch and the core has across-sectional dimension of about 0.0014 to about 0.00195 inch. Thecore has a density of about 9.9 g/cc or greater. The core is selectedform the group consisting of tungsten, tantalum, rhenium, iridium,silver, gold, bismuth and platinum. The outer member is selected fromsuperelastic alloys and precursors of superelastic alloys and stainlesssteel. The outer member is nitinol. The core is tantalum.

[0014] Another, particular aspect of the invention features aself-expanding tubular prosthesis device for use within the body formedof loosely interlocked knitted loops of a metal filament material. Thefilament is formed of an elastic metal capable of deflection withoutplastic deformation to produce a self-restoring force. The filamentmaterial is formed of an elastic metal outer member of extended lengthhaving high elasticity and an exposed outer surface, and a core of adifferent metal than the outer member, which core is secured within andsubstantially enclosed by the outer member. The device is capable ofreduction to a small size for introduction into the body lumen andexpandable by the internal restoring force to a sufficiently large sizeto engage the wall of the body lumen.

[0015] Various embodiments of this aspect as well as other aspects mayinclude the features already mentioned as well as one or more of thefollowing features. The core is about 25% or more of the cross-sectionaldimension. The core is between about 1 and 40%, e.g. about 28% or less,preferably 33% of the cross-sectional dimension. The core has a modulusof elasticity of about 500 GPa or less, such as about 200 GPa or less.

[0016] In another aspect the invention features a medical stent devicecapable of placement or manipulation in the body by means external ofthe body under guidance of a fluoroscope. The device is at least in partan elongated filament-form metal member adapted to be subjected toelastic deformation to enable the device to be forced into acharacteristic deformed configuration during a stage of use and toelastically self-recover from the deformation when deformation forcesare relieved. The filament-form metal member includes a core of a firstmetal of a first selected thickness and an intimately surrounding sheathof a second selected metal of a second thickness. The first metal is ahigh density metal that demonstrates characteristic relatively highradiopacity and the second metal is a lower density metal havingsubstantially more elasticity than the first metal. The combined effectof the selected thicknesses of the first and second metals in thefilament-form member serves to enhance the radio-opacity of thefilament-form member to provide improved fluoroscopic or x-rayvisualization of the filament-form member in the body while impartingsufficient elasticity to enable the filament-form member to elasticallyself-recover from its characteristic deformed configuration.

[0017] In another aspect, the invention features a tubularendoprosthesis formed of a metal member. The metal member has across-sectional thickness of about 0.015 inch or less, more preferably0.0075 inch or less, and is composed of at least two different metals,including an exposed outer metal having selected mechanical propertiesand an inner metal encompassed within the outer metal, the inner metalhaving a relatively high density compared to the outer metal forenhancing the radiopacity of the endoprosthesis.

[0018] In various embodiments of any of the aspects of the invention thefilament is formed by draw-forming techniques employing a large startingmember which has a core of metal of different properties than asurrounding metal shell.

[0019] The invention also includes methods for the use and constructionof the endoprostheses described.

[0020] Still other aspects of the invention will be understood from thefollowing description and from the claims.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

[0021] We first briefly describe the drawings.

DRAWINGS

[0022]FIG. 1 is a perspective view of a stent according to theinvention, while FIG. 1a is an enlarged view of adjacent loops of afilament knitted to form the stent;

[0023]FIG. 2 is a highly enlarged schematic cross-sectional view of thestent filament in FIG. 1a across the lines 22; while FIG. 2a is asimilarly enlarged longitudinal cross-sectional view of a portion of thefilament;

[0024]FIG. 3 is a schematic longitudinal cross-sectional view of a stentfilament according to FIG. 2 in a bent configuration;

[0025] FIGS. 4 to 4 b illustrate placement of a self-expanding stentaccording to the invention;

[0026] FIGS. 5 to 5 b illustrate placement of a plastically deformablestent according to the invention;

[0027]FIG. 6 is a graph of load as a function of displacement forseveral stent wires according to the invention.

DESCRIPTION

[0028] Referring to FIGS. 1 and 1a, an endoprosthesis stent 10 accordingto a preferred embodiment is adapted for use in the biliary tree andformed of an elastic filament 11 knitted into a mesh cylinder 12 thatextends to ends 14, 14′ coaxially along axis 13 over a working length L,about 4-6 cm and has a maximum expanded diameter, D, of about 9-10 mm.The knitting forms a series of loosely interlocked knitted loops (e.g.,as indicated by adjacent loops 16, 16′, FIG. 1a) which may slide withrespect to each other when the stent is radially compacted, for example,when delivered into the biliary duct on a catheter as further discussedbelow.

[0029] Referring to FIGS. 2 and 2a, the filament 11 is a wire-formmember that includes a longitudinal outer member 4 concentricallydisposed about a central core 8 which extends along an axis 6. Thelongitudinal member 4, having an outer diameter, d_(o) about 0.0052inch, is formed of a metal, such as nitinol, that exhibits desirableproperties, such as high elasticity and biocompatibility of its exposedouter surface 7. (The surface 7 may include a non-metal coating of,e.g., fluorocarbons, silicones, hydrophilic and lubricous biocompatiblematerials.) The core 8 having a diameter, dc, about 0.00175 inch,includes a metal, such as tantalum, with a density greater than thelongitudinal member 4 to enhance the radiopacity of the filament andthus the stent from which it is formed. The core 8 is bonded to andsubstantially enclosed by the outer member 4 such that the core does nothave any substantial exposed surface and therefore does not contact bodytissue when positioned within the body during use. As illustrated,preferably the core 8 is a continuous solid member in intimate contactwith and bonded to the interior portions of the outer member 4 withoutthe formation of substantial voids in the interface 10 between the coreand outer member. Preferably, the elastic properties of the filament 11are dominated by the elastic properties of the longitudinal member 4.The core material 8 enhances the radiopacity of the filament 11 butpreferably does not substantially affect the mechanical performance ofthe filament. One aspect of the present invention is that it has beendiscovered that a stent can be formed of a composite filament exhibitingsubstantially the elasticity properties of, for example, a solid elastic(used in linear range) or superelastic nitinol filament, (to form, forexample, a self-expanding stent), despite the presence of a dense, e.g.tantalum, core, and that the stent formed of the composite filament ismore radiopaque than a stent formed of a solid nitinol filament.

[0030] Referring to FIG. 3, the filament 11 is shown in a bent position,as it may be, for example when in use in a knitted stent device. Theinner and outer portions (I) and (O), respectively, experience a widerange of compression and tension, as the filament is bent such as duringknitting of the stent and during placement of the stent and in use. Anadvantage of the filament is that by positioning the radiopaque corematerial 8 near the axis 6, the range of tension and compression imposedon the core is relatively small and a wide latitude of stiff, dense,strong, and/or substantially radiopaque materials can be used whichotherwise might not be suitable for their response to bending or otherproperties.

[0031] Parameter Selection

[0032] Generally, the characteristics of the filament and thus theselection of the outer member and core metals, is based on the stentapplication. Particularly preferred uses of stents of the invention arefor the biliary tree (e.g. 8 -12 mm diameter, 2-8 cm length) such as thehepatic and pancreatic duct (e.g. 4-12 mm diameter), the urinary tractincluding the prostate and urethra (e.g. 14-15 mm, diameter; 3-4 cm,length) and the ureter (e.g. 3-4 mm diameter), and the vasculatureincluding the hepatic vasculature e.g. for the treatment of portalhypertension (e.g. 4-10 mm diameter; 5-6 cm, length), theneurovasculature (e.g. 1 mm, diameter) and the aorta (e.g. 20 mm,diameter) for example, for treatment of aneurysms or lesions. A filamentwith a larger outer diameter, d_(o), may be used for larger stents. Forexample, a filament outer diameter, d_(o), in the range of 0.008 inchmay be used in the case of an aortic stent.

[0033] Typically, the dimension of the core (d_(c)) is less than about50% (but typically greater than about 1%) of the outer diameter of thefilament, more preferably between about 40% and 25%, for example about33%. Embodiments of the stent having enhanced radiopacity are especiallyuseful in applications where the stent must be sized small with, forexample, a stent wire outer diameter (d_(o)) of less than about 0.015inch, e.g., even less than 0.0075 inch and for which less dense metalsare required for advantageous elastic properties. For example, inanother embodiment the filament is formed of nitinol with an outerdiameter, d_(o), of about 0.006 inch and a core of tantalum withdiameter, d_(c), at about 0.002 inch.

[0034] Referring to FIGS. 4 to 4 b, in embodiments such as thatdiscussed with respect to FIGS. 1 and 1a, stent 10 is a self-expandingstent that may be introduced to a body lumen 20 such as the biliarytree. The stent 10 is positioned on a catheter 24 which includes asleeve 26 to maintain the stent 10 in a relatively compact form (FIG.4). This is typically achieved by rolling the stent upon itself and apair of tiny mandrels that nip a portion of the wall of the stent andare rotated bodily together until the full circumference of the stent istaken up in the resulting rolled up form. In other cases, the stent maybe positioned coaxially over the catheter. The catheter is positionedwithin the lumen at the region corresponding to a tumor 22 and thesleeve 26 is removed from about the stent 10, for example, bywithdrawing axially in the direction of arrow 28, thus causing the stent10 to radially expand by release of its internal restoring force (FIG.4a). The internal restoring force is sufficient to dilate the lumen 20by pushing the tumor growth 22 radially outward (or in some casescompressing the occlusion against the lumen wall), thus opening apassage that allows unobstructed flow through the lumen and allowingremoval of the catheter 24 (FIG. 4b).

[0035] In another embodiment, the stent may be compressed in a tubehaving a distal axial opening and be pushed from the end of the tube,whereby it self-expands.

[0036] Referring now to FIGS. 5 to 5 b, the stent may also be aplastically deformable tube-like knitted structure 50. The individualfilament meshes interlock loosely in a looped pattern and during radialexpansion of the knitted structure loops forming the individual meshesare deformed beyond the elastic limits of the filament, resulting inplastic deformation of the filament. Stent 50 is placed over a balloon51 carried near the distal end of a catheter 52. The catheter 52 isdirected through a lumen 54, e.g., a blood vessel until the portioncarrying the balloon and stent reaches the region of the occlusion 53(FIG. 5). The stent 50 is then radially expanded beyond its elasticlimit by the admission of pressure to the balloon 51 and compressedagainst the vessel wall with the result that occlusion 53 is compressed,and the vessel wall surrounding it undergoes a radial expansion (FIG.5a). The pressure is then released from the balloon and the catheter iswithdrawn from the vessel (FIG. 5b).

[0037] The stent in the balloon expandable embodiment is preferablyformed with a dense radiopaque core formed of tantalum and an outermember formed of plastically deformable stainless steel. While the stentfilament is plastically deformable, the filament may be selected to haveelasticity sufficient to meet the needs of the particular vessel. Inaddition, self-expanding stents, such as discussed above with respect toFIG. 4 et seq. can be used with a balloon delivery system to aiddilatation of a body lumen.

[0038] In various embodiments, the metals used in the filament and theirconfiguration are selected to exhibit various desirable characteristics.For example, the relative dimension of the core and outer member and theparticular materials used for these elements may be selected based onthe desired over-all mechanical properties of the stent and the degreeto which x-ray visibility is to be enhanced, since the core affects themechanical properties of the filament compared to a solid filamentformed of the outer material, and the radiopacity is a function of thesum of the mass between an x-ray beam source and detector. A knittedstent with overlapping portions, may require less radiopaque material toprovide sufficient visibility. Similarly, the location of use in thebody may affect the amount of dense material needed for sufficientvisibility. The visibility of a device can be tested by known techniquessuch as ASTM Designation F640-79 “Standard Test Method for Radiopacityof Plastics for Medical Use”. In this test, the background densitieswhich may be encountered clinically are mimicked by an aluminum platepositioned over the stent having various thicknesses.

[0039] The properties of the outer member metal and core which may beconsidered include density, modulus of elasticity (in annealed andhardened states), biocompatability (primarily a factor for the materialof the outer longitudinal member), flexural stiffness, durability,tensile and compression strength, and the required radiopacity andresolution. In some cases, if desirable, the inner and outer metals maybe the same metal or metals of the same elemental composition that aredifferent metals because of e.g., different crystal structure or otherproperties.

[0040] In other embodiments of elastic filament members, the outermember is formed of a continuous solid mass of a highly elasticbiocompatible metal such as a superelastic or pseudo-elastic metalalloy, for example, a nitinol (e.g., 55% nickel, 45% titanium). Otherexamples of superelastic materials include, e.g., Silver-Cadmium(Ag—Cd), Gold-Cadmium (Au—Cd), Gold-Copper-Zinc (Au—Cu—Zn),Copper-Aluminum-Nickel (Cu—Al—Ni), Copper-Gold-Zinc (Cu—Au—Zn),Copper-Zinc (Cu—Zn), Copper-Zinc-aluminum (Cu—Zn—Al), Copper-Zinc-Tin(Cu—Zn—Sn), Copper-Zinc-Xenon (Cu—Zn—Xe), Iron Beryllium (Fe₃Be), IronPlatinum (Fe₃Pt), Indium-Thallium (In—Tl), iron-manganese (Fe—Mn)Nickel-Titanium-Vanadium (Ni—Ti—V), Iron-Nickel-Titanium-Cobalt(Fe—Ni—Ti—Co) and Copper-Tin (Cu—Sn). See Schetsky, L. McDonald, “ShapeMemory Alloys”, Encyclopedia of Chemical Technology (3rd ed.), JohnWiley & Sons, 1982, vol. 20. pp. 726-736 for a full discussion ofsuperelastic alloys. Preferably in some cases of elastic filamentmembers, nitinol or other highly elastic metal is employed as the outermember, in an arrangement in which it is never stressed beyond thestraight line portion of its stress strain curve. Other examples ofmetals suitable for the outer member include stainless steel or theprecursor of superelastic alloys. Precursors of superelastic alloys arethose alloys which have the same chemical constituents as superelasticalloys, but have not been processed to impart the superelastic propertyunder the conditions of use. Such alloys are further described inco-owned and co-pending U.S. Ser. No. 07/507,375, filed Apr. 10, 1990,by R. Sahatjian (see also PCT application US91/02420) the entirecontents of which is hereby incorporated by reference.

[0041] The core material is preferably a continuous solid mass, but mayalso be in a powder-form. Typically, the core includes a metal that isrelatively dense to enhance radiopacity. Preferably, the core metal hasa density of about 9.9 g/cc or greater. Most preferably, the core isformed of tantalum (density=16.6 g/cc). Other preferred materials andtheir density include tungsten (19.3 g/cc), rhenium (21.2 g/cc), bismuth(9.9 g/cc), silver (16.49 g/cc), gold (19.3 g/cc), platinum (21.45g/cc), and iridium (22.4 g/cc). Typically, the core is somewhat stifferthan the outer member. Preferably, the core metal has a low modulus ofelasticity, e.g., preferably below about 550 GPa, e.g., such as tantalum(186 GPa). Generally, a smaller difference between the modulus ofelasticity between the outer material and core results in a smallervariation of the modulus from that of the outer material in the filamentof the invention. For larger differences, a smaller core may be used soas to produce a filament in which the elastic properties are dominatedby the outer material.

[0042] The outer member and core may be in many cross-sectionalgeometric configurations, such as circular, square, triangular,hexagonal, octagonal, trapezoidal and the geometrical configuration ofthe core may differ from that of the longitudinal member. For example,the outer member of a filament may be rectangular in cross-section witha rectangular core or triangular or hexagonal in cross-section with acircular core. A stent filament may also take on the form of tubing witha lumen within the core extending along the axis. A stent filament mayalso include successive layers of less dense outer material and moredense core material to form a multi-composite system of three layers ormore from exterior to center. The core may extend intermittently alongthe axis in a desired pattern.

[0043] The filament may be a draw-form member formed, for example, bydrilling out the center of a relatively large rod of the outer membermaterial to form a bore, positioning a rod of core material in the bore,sealing the ends of the bore, e.g., by crimping, and drawing the formthrough a series of dies of decreasing diameter until the desired outerdiameter is achieved. The component may be heat treated to anneal,harden or impart superelastic properties. Other methods of formation arealso possible, e.g., by coating the core with the desired outer materialsuch as by electro- or electroless plating. The materials used in theouter member and core are also selected based on their workability forforming the filament, including factors such as machinability, forforming the longitudinal member into a tubular piece and the core memberinto a rod shaped piece, stability in gaseous environments at annealingtemperatures, properties related to drawing, welding, forging; swaging,the ability to accept coatings such as adhesives, polymers, lubricantsand practical aspects such as cost and availability.

[0044] As evident from the above, the stents of the invention arepreferably constructed by knitting a filament, most preferably on acircular knitting machine. Knitted metal stents are discussed, forexample, in Strecker, U.S. Pat. No. 4,922,905. It will be appreciatedthat the stent may be formed from a composite metal filament by othermeans such as weaving, crocheting, or forming the filament into aspiral-spring form element. It will further be appreciated that thecomposite filament may be incorporated within a stent formed fromconventional metal or non-metal materials (e.g. dacron in the case of anaortic graft) to contribute desirable properties such as strength and/orradiopacity. The stent may also be in the form of a metal memberconfigured as other than a filament-form, e.g., a composite sheet formmember in the form of a cuff or tube.

[0045] The following example is illustrative of a stent filament.

EXAMPLE

[0046] An elastic, radiopaque filament for use in a stent may be formedas follows. A 500 foot length of filament (0.0052 inch in diameter)having an outer member formed of a precursor of a nitinol (55% Ni/45%Ti) superelastic alloy and a core material of tantalum (0.00175 inch indiameter) is formed by drilling a 0.25 inch diameter bore in a 0.75 inchrod of the outer member material and providing in the drilled lumen atantalum member of substantially matched outer diameter. The rod ismechanically forged in a standard hot forging and rolling apparatus,then hammered such that no substantial voids between the core and outerlongitudinal member are present. One end of the rod is sealed and theopposite end is cold drawn longitudinally through a die to the finaldiameter. Initially, the outer member of the filament is the precursorof a superelastic alloy, i.e., it has not been heat treated to impartthe superelastic property under the anticipated conditions of use.

[0047] Referring to FIG. 6, load versus displacement curves areillustrated. (For clarity, curves C, D and A are offset, successively,0.025 inch on the x-axis.) Curve A illustrates the filament as discussedin the above paragraph prior to heat annealing which induces thesuperelastic property; the filament exhibits substantially linearelastic strain as a function of stress to a break point z. Curves B, C,D illustrate stress/strain curves after annealing the filament at 460°C. for 3 minutes, 5 minutes and 15 minutes, respectively. As thesecurves illustrate, the superelastic nature of the filament issubstantially preserved, as evidenced by the substantial plateaus (p) onthe stress/strain curve, despite the presence of the tantalum core. Alsoas illustrated, the stress at which constant displacement occursdecreases with increasing annealing, as would be expected with asuperelastic material. The mechanical properties of the filament,therefore, are dominated by the nitinol alloy, despite the presence ofthe tantalum core.

[0048] Referring to Table I, the modulus of elasticity and plateaustress calculated based on stress-strain measurements as above, arecompared for the filaments of the invention and a solid filament ofNi—Ti alloy. TABLE 1 % Charge in Cored Ni—Ti Ta Cored Ni—Ti FilamentDiameter .0038″ .0052″ — Area 1.134 × 10⁻⁵ in² 2.124 × 10⁻⁵ in² —(Modulus of Elasticity) Precursor 5,401,300 psi 7,373,865 psi  +27% 460°@ 3 mins 6,967,150 psi 6,657,000 psi −4.5% 460° @ 5 mins 5,381,160 psi5,721,100 psi +6.0% 460° @ 10 mins 5,139,310 psi — — 460° @ 15 mins5,143,960 psi 5,551,924 +7.4% Plateau Stress (loading) 460° @ 3 mins101,400 psi 94,174 −7.2% 460° @ 5 mins 89,056 psi 84,757 −4.8% 460° @ 10mins 79,357 psi — — 460° @ 15 mins 72,303 psi 75,339 psi +4.1%

[0049] As the results in Table I illustrate, the modulus of elasticityof the filaments of the invention was varied less than 30% compared tothe solid Ni—Ti filament. The plateau stress of the filaments of theinvention using a superelastic outer member was varied less than about10% compared to a solid Ni—Ti superelastic filament. The compositefilament formed as described exhibits about 30% or more enhanced x-rayvisibility over a filament of the same thickness formed of solidnitinol.

[0050] It will be understood that such filaments may be employed both ininstances where the superelastic property is employed, and in instanceswhere it is not (all stress within the straight line portion of thestress strain curve).

[0051] The visibility of a knitted stent formed from the filament wasgreater than a comparison stent using a solid nitinol filament of larger(0.006 inch) diameter, yet using the stent of the invention, the forceneeded for radial compression of the stent was reduced compared to thestent formed of the thicker nitinol filament. Thus, radiopacity of thestent was enhanced while mechanical properties were dominated by theouter member, nitinol. Placement of the stent, as-described above, canbe monitored by x-ray fluoroscopy.

[0052] Preferably, filaments as described, dominated by the mechanicalproperties of an outer member, such as nitinol, and exhibiting generallysatisfactory radiopacity have outer diameter (d_(o)) of about 0.008 to0.0045 inch with a core, for example of tantalum, with diameter (d_(c))of about 0.0014 to 0.00195 inch.

[0053] Other embodiments are in the following claims.

1. A tubular prosthesis device for use within the body comprised of ametal filament material formed of a metal outer member of extendedlength having an exposed outer surface, and a core within said extendedouter member comprising a different metal than said outer member, saidcore being secured within and substantially enclosed by said outermember, said device being capable of reduction to a small size forintroduction into said body lumen and expandable to a sufficiently largesize to engage the wall of said body lumen.
 2. The device of claim 1wherein said outer member and core are constructed such that saidendoprosthesis is elastic and capable of radial reduction in sizewithout plastic deformation to said small size for introduction to thebody and self-expandable by an internal elastic self-restoring force tosaid large size for engaging said wall of said lumen.
 3. The device ofclaim 1 wherein said outer member and core are such that theendoprosthesis is plastically deformable and formed into said small sizefor introduction into the body and expandable by plastic deformation tosaid large size for engaging the wall of said lumen.
 4. The device ofclaim 2 or 3 wherein said device is formed into said tubular shape byknitting into loosely interlocked loops of said filament.
 5. The deviceof any one of claims 1, 2 or 3 wherein said metal of said core has adensity greater than said metal of said outer member of said device. 6.The device of claim 5 wherein said cross sectional dimension of saidfilament is about 0.015 inch or less.
 7. The device of claim 6 whereinsaid cross-sectional dimension of said filament is about 0.006 to about0.0045 inch and said core has across-sectional dimension of about 0.0014to about 0.00195 inch.
 8. The device of claim 7 wherein said core has adensity of about 9.9 g/cc or greater.
 9. The device of claim 8 whereinsaid core is selected form the group consisting of tungsten, tantalum,rhenium, iridium, silver, gold, bismuth and platinum.
 10. The device ofclaim 9 wherein said outer member is selected from superelastic alloysand precursors of superelastic alloys and stainless steel.
 11. Thedevice of claim 10 wherein said outer member is nitinol.
 12. The deviceof claim 11 wherein said core is tantalum.
 13. A self-expanding tubularprosthesis device for use within the body comprised of looselyinterlocked knitted loops of a metal filament material formed of anelastic material capable of deflection without plastic deformation toproduce a self-restoring force, said filament material comprising anelastic metal outer member of extended length and an exposed outersurface, and a core comprising a different metal than said outer member,said core being secured within and substantially enclosed by said outermember, said device being capable of reduction to a small size forintroduction into said body lumen and expandable by said internalrestoring force to a sufficiently large size to engage the wall of saidbody lumen.
 14. The device of claim 13 wherein said core is about 1 to40% of said of the cross-sectional dimension of said filament.
 15. Thedevice of claim 14 wherein said core is about 25% or more of saidcross-sectional dimension.
 16. The device of claim 15 wherein said coreis about 33% of said cross-sectional dimension.
 17. The device of claim14 wherein said core has a modulus of elasticity of about 500 GPa orless.
 18. The device of claim 15 wherein said core has a modulus ofelasticity of about 200 GPa or less.
 19. The device of claim 14 whereinsaid core has a density of about 9.9 g/cc or greater.
 20. The device ofclaim 19 wherein said core is selected form the group consisting oftungsten, tantalum, rhenium, iridium, silver, gold, bismuth andplatinum.
 21. The device of claim 20 wherein said outer member isselected from the group consisting of superelastic alloys and precursorsof superelastic alloys and stainless steel.
 22. The device of claim 21wherein said outer member is nitinol.
 23. The device of claim 22 whereinsaid core is tantalum.
 24. The device of claim 13 or 23 wherein saidcross sectional dimension of said filament is about 0.015 inch or less.25. A medical stent device capable of placement or manipulation in thebody by means external of the body under guidance of a fluoroscope, saiddevice comprised at least in part of an elongated filament-form metalmember adapted to be subjected to elastic deformation to enable thedevice to be forced into a characteristic deformed configuration duringa stage of use and to elastically self-recover from said deformationwhen deformation forces are relieved, said filament-form metal membercomprised of a core of a first metal of a first selected thickness andan intimately surrounding sheath of a second selected metal of a secondthickness, said first metal being a high density metal that demonstratescharacteristic relatively high radiopacity and said second metal being alower density metal having substantially more elasticity than said firstmetal, the combined effect of the selected thicknesses of said first andsecond metals in said filament-form member serving to enhance theradio-opacity of said filament-form member to provide improvedfluoroscopic or x-ray visualization of said filament-form member in thebody while imparting sufficient elasticity to enable the filament-formmember to elastically self-recover from its characteristic deformedconfiguration.
 26. The medical device of claim 25 wherein saidfilament-form metal member comprises a draw-form.
 27. The medical deviceof claim 26 wherein said second metal is nitinol.
 28. The medical deviceof claim 27 wherein said high density metal is tantalum.
 29. A tubularendoprosthesis device for use within the body comprised of a metalmember formed in tubular shape, wherein said metal member has across-sectional thickness of about 0.015 inch or less and is composed ofat least two different metals, including an exposed outer metal havingselect mechanical properties and an inner metal encompassed within saidouter metal, said inner metal having a relatively high density comparedto said outer metal for enhancing the radiopacity of saidendoprosthesis.
 30. The tubular prosthesis device of claim 29 whereinsaid metal member has a cross-sectional thickness of about 0.0075 inchor less.